Testing for carbonyls Flashcards
,Main test to see if a C=O bond is present
2,4-DNPH test
What does 2,4-DNPH have a positive test for?
Both aldehydes and ketones
Conditions for 2,4-DNPH test
Concentrated sulfuric acid
dissolved in methanol
Warmed
Positive result for 2,4-DNPH test
A bright orange precipitate is formed which are crystals
What can be done after the 2,4-DNPH test?
Purify crystals by recrystallisation
Complete melting point test to find MP
Compare to data booklet values of MP to identify the carbonyl we had
Does the 2,4-DNPH test differentiate aldehydes from ketones?
NO after a positive result for 2,4-DNPH test we can complete after tests to differentiate
3 tests that give a positive result for aldehydes but NOT ketones
Tollens’ reagent
Fehling’s/ Benedict’s solution
Acidified dichromate (VI) ions
Why can the 3 tests distinguish an aldehyde from ketone by giving a positive result for the aldehyde?
Because aldehydes can be oxidised again to carboxylic acids whereas ketones can only be oxidised by breaking C-C bonds
Positive result = shows substance was oxidised (aldehyde)
How to represent oxidising agent?
[O]
What is Tollens’ reagent?
Colourless silver nitrate solution dissolved in aqueous ammonia
Complex ion [Ag(NH3)2]+
Another name for Tollen’s reagent test
Silver mirror test
Conditions for Tollens’ reagent test
Heat the silver nitrate solution in aqueous ammonia with an aldehyde
Oxidising agent in Tollens’ reagent
Ag+ ions are reduced to Ag
And oxidises the aldehyde
What are aldehydes oxidised to when testing for them?
Carboxylic acids
What observation indicates a positive result for Tollens’ reagent?
A silver mirror formed which coats test tube as Ag+ ions were reduced to Ag atoms
What is Fehling’s solution?
Cu 2+ ions dissolved in NaOH
What is Benedict’s solution?
Cu 2+ ions dissolved in Na2CO3 solution
Oxidising agent in Benedict’s/Fehling’s solution
Cu 2+ ions that are reduced to Cu2O (Cu 1+ ions)
Observation for positive result of Benedict’s/Fehling’s test
Colour change blue solution to form red precipitate of copper oxide
Word equation for Fehling’s/Benedict’s?
Aldehyde + [O] ——-> Carboxylic acid + copper oxide
What is Acidified dichromate ions?
H+/Cr2O7 2- ions
Oxidising agent for acidified dichromate test?
Cr2O7 2-
The Cr 6+ ions are reduced to Cr 3+ ions
Observation for positive result acidified dichromate ions test?
Colour change orange to green
Iodoform test reagent
Iodine in presence of cold aqueous hydroxide alkali
What does iodoform give a positive result for?
Methyl carbonyl groups, the carbonyl group is bonded to a methyl group which is CH3
O=C-CH3
What molecules give a positive result for the iodoform test?
Ethanal
Any ketone which C=O is bonded to the 2nd carbon so there is a methyl group to be the end of the chain
Positive result for iodoform test
Yellow precipitate of CH3I
And antiseptic smell
Why do only methyl carbonyl groups give a positive iodoform test result?
Because a yellow precipitate of CH3I was formed
